Method and apparatus for controlling the anodization of film resistors



Jan. 23, 1968 H. R. KAISER 3,365,379

METHOD AND APPARATUS FOR CONTROLLING THE ANODIZATION OF FILM RESISTORS Filed April 15, 1965 2 24 PHASE DETECTOR F'LTER FIG. 1

I FIG. 2

LU 2 q 200- p.- 2 loo- E 0 I0 20 so. 40 so so E TOR TIME SECONDS HAROLD R. KAISER BY 2: Z

Agent United States Patent 3,365,379 METHOD AND APPARATUS FOR CONTROLLING THE ANODIZATION OF FILM RESISTORS Harold R. Kaiser, Los Altos, Califi, assignor to Lockheed Aircraft Corporation, Burbank, Calif. Filed Apr. 13, 1965, Ser. No. 447,724 14 Claims. (Cl. 20438) The present invention relates to a method and apparatus for controlling the anodization of film resistors, and more particularly to an improved method and apparatus for controlling the anodization of film resistors for use in microminiaturized electronics.

As a result of the growing importance and interest in the microminiaturization of present day electronics, considera'ble effort is being expended in the search for improved techniques for the manufacture of both active and passive electronics components. Considerable attention has been given to thin film resistors because of their inherent simplicity, compactness, and economy of manufacture. However, the provision of high quality film resistors for miniaturization has proven to be a diflicult problem, requiring expensive and complicated methods and apparatus, for the manufacture of such devices. Thus, it is more important than ever that relatively simple and inexpensive methods and apparatus be made available for providing film resistors of high accuracy and stability.

In the prior art, anodization of metal films to produce resistors has been accomplished essentiallyby continuously measuring the resistance of the thin metal film being processed and then terminating the process once the desired resistance has been reached. The accuracy with which such resistors may be made with prior art techniques was predicated on the fact that there exists a linear relationship to the anodizing voltage and the thickness of the anodized film. This relationship, however, is not one that can always be predicated and does not always exist to the extent necessary to assure satisfactory control of the anodizing process.

In accordance with the present invention, an improved method and apparatus has been developed that greatly reduces the complexity of the apparatus and cost which was heretofore required to make film resistors of high accuracy and stability in mass production. The present invention is essentially an improvement in existing techniques for converting thin metallic resistive films, such as titanium, tantalum, zirconium, hafnium, aluminum, and niobium, into highly stable films of high resistivity by means of anodizing techniques utilizing an AC bridge as the device for the controlling anodizing current, and the resistance measurement. More particularly, it has become possible in accordance with the present invention, to continuously anodize thin resistive films during the conversion process and to halt the process at the precise moment when the film reaches the desired resistivity to produce resistors within tolerances of less than :1 percent and in most cases substantially less than the one percent.

Accordingly, it is the broad objective of this invention to provide an improved method and apparatus for making precision film resistors.

A more specific objective of this invention is to provide a method and apparatus for accurately terminating the conversion process when the resistivity of the film reaches the desired resistivity.

Another object of this invention is to provide a method and apparatus for the conversion of resistive films which is extremely accurate and may be adapted to mass production techniques.

The specific nature of the invention, as well as the ob.- jectives, uses, and advantages thereof, will be more closely 3,365,379 Patented Jan. 23, 1968 ice understood from the following description and from the accompanying drawings in which:

FIGURE 1 is a circuit and schematic diagram of the apparatus for automatically controlling the conversion process of thin metal films of high resistivity, when the resistivity of the film reaches a predetermined value.

FIGURE 2 is a plot of resistance versus time and represents the characteristic curve of the resistive element which determines the rate at which the anodizing process proceeds and determines the final value of resistivity of the resistor film.

The objects and advantages derived from the present invention are accomplished by utilizing an alternating current bridge with the resistive metal film to be anodized as one leg of the bridge having essentially no resistance as the conversion process is commenced. A variable resistor as another leg of the bridge is employed and is connected at one end to the resistive metal film to be anodized. The variable resistor covers a range of resistance of zero to some prescribed value and in a specific embodiment where the variable resistor is a helipot or the like it may be varied continuously during the anodizing process by means of a constant speed motor connected mechanically thereto. The entire resistance range is covered by the variable resistor during a prescribed period of time. It should be noted at this point that it is possible to utilize a variable resistance device such as a ramp generator and the like, employing a thyratron to determine the magnitude of resistance and the rate of change thereof during a preselected period of time. The bridge is completed by two other legs, one having a fixed resistor and the other a variable resistor which may be set for various values before the anodizing process is commenced and determines in part the final value of the resistor as the bridge is continuously rebalanced during the anodizing process.

With reference to the drawings, FIGURE 1 depicts an alternating bridge circuit generally designated 10, having four legs defined by points A, B, C and D, an alternating current source 12 in the form of a Power Supply electrically connected to points A and C of the bridge, an amplifier 14 connected at one end to point D of the bridge circuit 10 and connected at the other end in series with a phase-sensitive detector 16, a low pass band filter 18, and the control bridge 20 of an electron tube 22. An output plate 24 of the electron tube 22 is connected to an anodizing electrode 26 which forms part of direct current conversion circuit for anodizing a thin metal film 28. Both film 28 and electrode 26 are immersed in an anodizing solution 30 in a container 32, which is illustrated by a broken line. The circuit is completed by grounding point B of the A.C. bridge and connecting the A.C. source 12 to the phase detector 16. A control switch 42 is connected between the power supply 12 and point C of the bridge circuit which actuates the process upon being closed.

Referring now more specifically to alternating current bridge 10, leg AB contains a variable resistor 31, which may be set at a predetermined value and stays fixed dur ing the anodizing process, so as to have a definite relationship to the adjoining leg between points B and C, Which has a fixed resistor 34. Between points C and D is a variable resistor 36, which may be any suitable potentiometer or helipot having a resistance in the range of 0 to some preselected value.

The helipot 36 is varied from 0 to its maximum value by a suitable constant speed motor 38 mechanically linked or connected thereto, as designated by the broken line 40.

The last leg of the bridge D-A contains the thin metallic film 28 which is ultimately converted to a resistor. As shown in FIGURE 1, the film 28 is immersed in an anod izing solution 30, and has a resistance value of theoretically zero when the anodizing process is commenced, but

in practice has a small ferrite value. The resistance values of the legs of the bridge are chosen such that the relationship between resistances 31 (R and 34 (R is proportioned to the resistances 36 (R and 28 (R Stated in another manner, the relationship is as follows:

From the foregoing Equation 1 it can be seen that R /R has a fixed ratio, and thus the final value of the resistance R will also be proportional to the value of R when the anodization process is terminated as a result of the AC. bridge being rebalanced.

Operation of the apparatus is commenced by closing the switch 42 which applies an energizing alternating current to the bridge and also causes an anodizing current (I to flow in the conversion circuit. The bridge is initially unbalanced owing to the fact that a resistance of R is not exactly zero, since in practice a resistor 44, for example, as shown in FIGURE 1, is placed in parallel with the potentiometer, R

The bridge is continuously being unbalanced during the anodization process, as a result of the resistance of the potentiometer or helipot 36 being varied until its maximum value is reached, which is determined by the speed of the connected motor 38 which also prescribes length of time it takes to cover the range of the helipot.

Stated in a different manner, during the process, the anodizing current (I of the conversion circuit changes continuously in an attempt to rebalance the bridge. Thus balancing of the AC. bridge is accomplished by the action of the anodizing circuit in response to an unbalanced condition produced in the bridge circuit by the variable resistance 36 of leg OD. The correct phase of the anodizing current is determined by the phase detector circuit 16 in conjunction with the reference signal to the phase detector from the AC. source 12. When the circuit is drawing too large or too small a current the phase detector function to cause the correct current to flow.

Thus, it can be seen that the system operates essentially on an AC. balancing bridge principle with the exception that the bridge is being continually unbalanced by a change of resistance of one of the legs, R in parallel with R in this illustrative embodiment, in response to the rotation of the constant speed motor mechanically linked thereto. The rebalancing process of the bridge is a continuous one, and the magnitude of the anodizing current (I depends upon the magnitude of the resistance unbalance in the bridge caused by a change in metal film 28.

Reference to FIGURE 2 illustrates that the resistance leg DC (R in parallel with R is varied the greatest during the initial portion of a cycle and tapers off rapidly as the resistance in leg A-D (R reaches the end of the cycle. Thus, it can be seen that the magnitude of anodizing current would be greatest during the early portion of the cycle and is considerably less during the latter portion thereof. This slowing down of the anodizing rate accounts for the close tolerance which may be achieved with the present invention for producing high precision resistors. In addition, it should be noted from FIGURE 2 that the cycle illustrated is a rather short one, on the order of one minute duration. It should also be noted at this point that the speed with which an unanodized film may be converted to a resistor is limited only by speed with which the constant speed motor can program the variable resistance (R through the resistance range and the ability of the anodizing circuit to control the anodizing current (I at such a rate which will enable it to follow the change in resistance as it is varied during the particular cycle.

An example of the type of resistor that may be obtained through the use of the present invention may more readily be understood by an illustrative example. The object in this illustrative example was to convert a thin titanium film having an initial resistance value of about 40 to 50 ohms (15 to 20 ohms per square) to a resistor of 2330 ohms having a tolerance of about :1 percent. Referring to FIGURE 1, the legs of bridge 10 was set up as follows:

R31=2330 Ohms R =500 ohms :.05 percent R =1000 ohms (having a 1000 ohm :1 percent fixed resistor (R in parallel) R =40 to 50 ohms (initially) The resistance (R of leg C-D in the bridge was a 0 to 1000 ohms helipot having ten-turns and was connected by mechanical linkage to the constant speed motor 38 which turns at ten revolutions per minute. A fixed resistor 44 (R of ohms :1 percent is connected in parallel with R producing an initial resistance greater than zero. Resistor 31 (R is set to the value of 2330 ohms, by using the resistance setting on a General Radio 510 decade resistance box having five resistance ranges. Resistance 34 (R is a fixed resistance of 500 ohms :05 percent.

Referring to the Equation 1 it can be seen that resistor (R will reach a final resistance value in accordance therewith. The direct current (I is controlled by the series connected amplifier 14, phase detector 16, and a reference signal 46 from the AC. power supply 12. This arrangement enables the direct current circuit to increase or decrease the current as required so as to maintain substantially uniform control of the anodizing process.

Operation of the circuit commences when the switch 42 is closed and the motor 38 is turned on. As indicated by a broken-line 48 the motor 38 is started at substantially the same time as the switch 42 is closed. The anodizing solution utilized may be a suitable solution known in the prior art and is not considered part of the present invention. However, it should be noted that the anodizing solution must be sensitive and capable of rapid response to the changes in the anodizing current levels. In this example the voltage range employed was 300 volts and the initial anodizing current was about 2.0 milliamperes, and the current dropped to the range of 30-50 microamperes during the last 30 seconds of the cycle.

The present example was concluded with the fabrication of a resistor (R having a value of 2345 ohms, which is less than a plus 1 percent tolerance of the resistance setting of (R 2330 ohms.

In closing it is useful to summarize some of the advantages of the present invention. One such advantage arises from the fact that the resistance value of the thin resistive film may be converted to a desired value in a relatively short period of time, on the order of a minute, producing resistors having a tolerance of less than :1 percent by an automatic control arrangement. Another advantage arises from the fact that apparatus of the invention, namely the bridge circuit, control circuit and motor driven helipot are combined in a manner which is readily adaptable to mass reproduction techniques.

It is to be understood that the above described embodiment is only illustrative of the principle applicable in the present invention. Numerous other applications and modifications may be defined by those skilled in the art without departing from the scope of the invention. Accordingly, it is to be understood that the present invention is limited only by the spirit and scope of the specification, drawings, and appended claims.

What is claimed is:

1. The method of producing a resistor comprising the steps of coating an insulation substrate with a film of a metal capable of anodically forming a dielectric coating thereon and providing two direct electrical contacts to said film, positioning said contacts so they will serve for potential terminals of an alternating current bridge, one

of said contacts being utilized for measuring any anodizing current of said film and will not directly touch an anodizing electrolyte placed against the exposed face of said film, passing a direct current anodizing between said film and an electrode immersed in said electrolyte and in electrical proximity with said film the magnitude of said anodizing current being determined by the magnitude and electrical phase of a voltage unbalance in the bridge circuit, varying the resistance in a leg of the bridge circuit having one of its terminals connected to a common junction with one of said contacts of said films, said variable resistance being varied precisely by a constant speed motor mechanically linked thereto over a preselected range of resistance during a prescribed period of time so as to reach a desired resistance value at the end of said prescribed period of time whereby said anodizing current is terminated owing to rebalancing of the bridge.

2. In an anodizing apparatus including an alternating current potential bridge circuit comprising A.C. input and output circuit terminals, 21 first variable resistor forming a first arm of the bridge, a constant speed motor mechanically linked to said first variable resistor for varying the resistance thereof at a preselected rate of change during a preselected period of time, a fixed resistor forming a second arm of the bridge and connected at one end thereof to said first variable resistor, a second variable resistor forming a third arm of the bridge, and connected at the other end of said fixed resistor, a fourth arm of the bridge connected between said first and second variable resistors to complete the bridge, said fourth arm of the bridge being adapted to contain a thin conductive film capable of being anodized to vary the resistance of said fourth arm, a series connected amplifier, a phase sensitive detector, a filter and a grid of an electron tube which are in turn connected to the bridge circuit by said series connected amplifier having an anodizing electrode connected to output plate of the electron tube for passing an anodizing current through said thin conductive film, said anodizing current being responsive to said second variable resistor and to said series connected amplifier, phase detector, filter, electron tube and an alternating current power source which provides power to the bridge and a reference signal to said phase sensitive detector.

3. An apparatus including an alternating current potential bridge having an alternating current input and unbalance detection output terminals, a first variable resistance forming one arm of said bridge, a constant speed motor mechanically linked to said first variable resistor for varying the resistance thereof at a preselected rate of change during a prescribed period of time, a second arm of the bridge connected at one end to said first resistor to thereby form said unbalanced detection terminal, said second arm of the bridge being adapted to contain a thin conductive film capable of being anodized to vary the resistance of said second arm, and two series connected resistance elements forming the other legs of the bridge the ends of which are connected to the respective ends of said second arm of the bridge and the first resistor thereby forming the input terminals of said bridge, an alternating current source connected to said input terminals and means for generating a direct current for anodizing said conductive film and for adjusting said anodizing current passed through said conductive film and an anodizing electrode disposed in an anodizing electrolyte placed against the exposed surface of said conductive film, said bridge being rebalanced by said anodizing current with respect to changes in the resistance of said first resistor as said mechanically linked rnotor rotates.

4. The apparatus defined in claim 3 wherein said means for generating a direct current for anodizing said conductive film includes a series connected amplifier, phase detector, filter, electron tube, an anodizing electrode, and a phase reference signal from said alternating current source fed to said phase detector.

5. The apparatus defined in claim 4 wherein the phase of the anodizing current is held in phase with the current of the bridge to ensure rebalancing of the bridge as the resistance of said first resistor is varied.

6. The apparatus defined in claim 3 wherein said prescribed period of time is on the order of 60 seconds as the resistance of said first variable resistance is varied from zero to a predetermined value.

7. The apparatus defined in claim 6 wherein said predetermined value is on the order of 500 ohms.

8. The apparatus defined in claim 3 wherein said first variable resistor is a helipot having a resistance which varies from zero to 500 ohms.

9. The apparatus defined in claim 3 wherein said first variable resistor consists of a parallel connected fixed resistor and a variable resistor.

10. The apparatus defined in claim 3 wherein said means for generating a direct current for anodizing said conductive film includes means for rectifying an alternating current.

11. A method of producing a resistor comprising the steps of coating an insulative substrate with a film of metal capable of anodically forming a dielectric coating thereon and providing two electrical contacts to said film, positioning said contacts so they will serve for potential terminals of an alternating current bridge, one of said contacts being utilized for measuring an anodizing current of said film and will not directly touch an anodizing electrolyte placed against the exposed face of said film, passing a direct current anodizing current between said film and an electrode immersed in said electrolyte and in electrical proximity with said film, the magnitude of said anodizing current being determined by the magnitude and electrical phase of a voltage unbalanced in the bridge circuit, varying the resistance in a leg of the bridge circuit having one of its terminals connected to a common junction with one of said contacts of said film, said variable resistance being varied precisely by a variable speed device over preselected range of resistance during a preselected period of time so as to reach a desired resistance value at the end of said prescribed period of time whereby said anodizing current is terminated owing to rebalancing of the bridge.

12. The method defined in claim 11 wherein said variable resistance device is further defined by a ramp generator employing a thyratron to determine the magnitude of resistance and the rate of change thereof during a preselected period of time.

13. An anodizing apparatus including an alternating current potential bridge circuit comprising A.C. input and output circuit terminals, a first variable resistance device forming a first arm of the bridge, a second arm of the bridge connected at one end thereof to said first variable resistance device, said second arm of the bridge being adapted to contain a thin conductive film capable of being anodized to vary the resistance of said second arm, a second variable resistor forming a third arm of the bridge and connected to the other end of said second arm, a fixed resistor forming a last arm to complete the bridge connected at one end to said first variable resistance device and at the other end to said second variable resistor, a series connected amplifier, phase sensitive detector, filter and DC. current rectifying device which are in turn connected to the bridge by said series connected amplifier having an anodizing electrode connected to the output of said current rectifying device passing an anodizing current through said thin conductive film, said anodizing current being responsive to said second variable resistor and to said series-connected amplifier, phase detector, filter, current rectifying device and an alternating current power source which provides power to the bridge and a reference signal to said phase sensitive detector.

14. The apparatus defined in claim 13 wherein said first variable resistance device is further defined by a ram generator employing a thyratron to determine the mag- 7 8 nitude of resistance and the rate of change thereof during 3,282,821 11/ 1966 Cistola 204228 a preselected period of time. 3,341,444 9/ 1967 La Chapelle 204228 3,341,445 9/1967 Gerhard 204---228 References Cited 5 HOWARD S. WILLIAMS, Primary Examiner.

UNITED STATES PATENTS 3,148,129 9/1964 Basseches et a1. 204228 XR 3,159,556 12/1964 McLean et a1. 20456 XR G. KAPLAN, Assistant Examiner. 

1. THE METHOD OF PRODUCING A RESISTOR COMPRISING THE STEPS OF COATING AN INSULATION SUBSTRATE WITH A FILM OF A METAL CAPABLE OF ANODICALLY FORMING A DIELECTRIC COATING THEREON AND PROVIDING TWO DIRECT ELECTRICAL CONTACTS TO SAID FILM, POSITIONING SAID CONTACTS SO THEY WILL SERVE FOR POTENTIAL TERMINALS OF AN ALTERNATING CURRENT BRIDGE, ONE OF SAID CONTACTS BEING UTILIZED FOR MEASURING ANY ANODIZING CURRENT OF SAID FILM AND WILL NOT DIRECTLY TOUCH AN ANODIZING ELECTROLYTE PLACED AGAINST THE EXPOSED FACE OF SAID FILM, PASSING A DIRECT CURRENT ANODIZING BETWEEN SAID FILM AND AN ELECTRODE IMMERSED IN SAID ELECTROLYTE AND IN ELECTRICAL PROXIMITY WITH SAID FILM THE MAGNITUDE OF SAID ANODIZING CURRENT BEING DETERMINED BY THE MAGNITUDE AND ELECTRICAL PHASE OF A VOLTAGE UNBALANCE IN THE BRIDGE CIRCUIT, VARYING THE RESISTANCE IN A LEG OF THE BRIDGE CIRCUIT HAVING ONE OF ITS TERMINAL CONNECTED TO A COMMON JUNCTION WITH ONE SAID CONTACTS OF SAID FILMS, SAID VARIABLE RESISTANCE BEING VARIED PRECISELY BY A CONSTANT SPEED MOTOR MECHANICALLY LINKED THERETO OVER A PRESELECTED RANGE OF RESISTANCE DURING A PRESCRIBED PERIOD OF TIME SO AS TO REACH A DESIRED RESISTANCE VALUE AT THE END OF SAID PRESCRIBED PERIOD OF TIME WHEREBY SAID ANODIZING CURRENT IS TERMINATED OWING TO REBALANCING OF THE BRIDGE.
 3. AN APPARATUS INCLUDING AN ALTERNATING CURRENT POTENTIAL BRIDGE HAVING AN ALTERNATING CURRENT INPUT AND UNBALANCE DETECTION OUTPUT TERMINALS, A FIRST VARIABLE RESISTANCE FORMING ONE ARM OF SAID BRIDGE, A CONSTANT SPEED MOTOR MECHANICALLY LINKED TO SAID FIRST VARIABLE RESISTOR FOR VARYING THE RESISTANCE THEREOF AT A PRESELECTED RATE OF CHANGE DURING A PRESCRIBED PERIOD OF TIME, A SECOND ARM OF THE BRIDGE CONNECTED AT ONE END TO SAID FIRST RESISTOR TO THEREBY FORM SAID UNBALANCED DETECTION TERMINAL, SAID SECOND ARM OF THE BRIDGE BEING ADAPTED TO CONTAIN A THIN CONDUCTIVE FILM CAPABLE OF BEING ANODIZED TO VARY THE RESISTANCE OF SAID SECOND ARM, AND TWO SERIES CONNECTED RESISTANCE ELEMENTS FORMING THE OTHER LEGS OF THE BRIDGE THE ENDS OF WHICH ARE CONNECTED TO THE RESPECTIVE ENDS OF SAID SECOND ARM OF THE BRIDGE AND THE FIRST RESISTOR THEREBY FORMING THE INPUT TERMINALS OF SAID BRIDGE, AN ALTERNATING CURRENT SOURCE CONNECTED TO SAID INPUT TERMINALS AND MEANS FOR GENERATING A DIRECT CURRENT FOR ANODIZING SAID CONDUCTIVE FILM AND FOR ADJUSTING SAID ANODIZING CURRENT PASSED THROUGH SAID CONDUCTIVE FILM AND AN ANODIZING ELECTRODE DISPOSED IN AN ANODIZING ELECTROLYTE PLACED AGAINST THE EXPOSED SURFACE OF SAID CONDUCTIVE FILM, SAID BRIDGE BEING REBALANCED BY SAID ANODIZING CURRENT WITH RESPECT TO CHANGES IN THE RESISTANCE OF SAID FIRST RESISTOR AS SAID MECHANICALLY LINKED MOTOR ROTATES. 